Warzone: "any clear, flat area" - but theres a drawing in the book that looks like 120x180cm

So the most common size is 120x180, or 6 small modules of 60x60cm.
Always remember, you need a surface where you can put your gaming modules on.
A common ping-pong Table is 274 x 152,5 cm, a paste table is roughly 300 x 60 cm, the ordinary kitchen table 120 x 80 cm.
When you're placing the 60x60cm modules on a kitchen table they're overlapping the table 20 to 30cm at each side. Keep that in mind when you're placing heavy objects near the edge of the gaming table.
Hard polystyrene boards are about 120x60cm. You can either place them under your modules to provide more stability or build your gaming table directly of those boards, 3 of them make the common gaming table size.